A well known method of making steel pipe is to helically weld steel skelp. Steel skelp is an elongated metal sheet, and is usually provided in large coiled rolls. The sheet is continuously fed to a pipe former where the sheet is helically wound into the form of a pipe and the edges of the sheet are welded together. The preferred weld is a butt weld, with one edge of the sheet abutting the opposing edge of a portion of the sheet already formed in the shape of a pipe. As a practical matter, the edges of the metal sheet are welded together as the shape of the pipe is formed. In order to form a helical pipe, the sheet has to be fed to the pipe former in a direction at an angle to the axis of the formed pipe. This angle is generally referred to as the helix angle. The width of the sheet, the helix angle, and the distance of the sheet from the axis of the pipe determine the diameter of the pipe.
Problems in the manufacture of pipe using such a method may occur due to irregularities in the steel skelp. Steel skelp is not a precision product. The width of the skelp may change throughout the length of the coil. The unrolled skelp also may not be perfectly linear, the skelp often contains camber which causes it to curve in a snake-like fashion. Camber and sheet width changes increase the difficulty in maintaining pipe diameter and also cause problems in the welding together of the sheet edges. Welding blowthrouah may occur if there is a gap between the abutting edges of the sheet at the point of welding, even if the gap is slight. Eliminating the gap between the sheet edges as they are being welded avoids the problem of welding blowthrough.
Methods to minimize weld gaps are known. One method is to shear the edges of the skelp coil to a constant width in a manner that eliminates most of the camber. This method, however, results in a material loss of approximately 3 percent. Another method is to determine the extent of the gap, if any, between abutting edges of the sheet. If a gap is detected, the helix angle is adjusted to close the gap. The leading edge of the incoming sheet, however, may also be offset radially from the pipe. The edges of the sheet may exhibit offset because the incoming section of the sheet to the weld point is not properly aligned. The offset may also occur because the edges of the sheet are being pressed together with undue pressure at the weld point. Such undue pressure generally causes the incoming sheet to climb or move radially outwardly with respect to the formed pipe, resulting in offset. The extent of the offset can be monitored and the helix angle adjusted to eliminate the offset. Doing so, however, may have unintended consequences with respect to pipe diameter. Therefore, it is desirable to minimize weld gap problems and offset, yet still maintain correct pipe diameter.